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Table 5. Average concentrations of dissolved-inorganic nitrogen and dissolved orthophosphate as phosphorus and corresponding ratios for site HEW2 at Lake Henry ImgIL, milligram'i per liter; <. less thanl Sampling date, 1987 DlssolYed~lnorganlc nitrogen (mgIL) May I June 29 August 19 October 6 0.170 .219 <.034 <.056 Dissolved orthophosphate 8S phosphorus (mgIL) <0.001 .002 .004 .037 Ratio of dissolved-Inorganic nitrogen to dissolved orthophosphate as phosphorus <170:1 110:1 <8.5:1 <1.5:1 Dissolved-solids concenlrations in Lake Henry ranged from 598 to 704 mglL and were smallest on June 29 and largest on May I and October 6 (table 16). As indicated by samples collected at site HEW2 near the lake surface and near the lake bollom and from specific-conductance measurements made at the other sites in Lake Henry, no substantial vertical or areal variations in dissolved-solids concentrations occurred on the sampling dates. Dissolved-solids concentrations (in mglL) for Lake Henry can be approximated from specific-conductance measurements, as indicated from eight samples collected during the sampling period, by multiplying the specific conductance by 0.68. On the basis of the analysis of eight water sam- ples collected at site HEW2, the predominant cation in Lake Henry is calcium and the predominant anion is sulfate. Calcium averaged about 47 percent of the cat- ions, and sulfate averaged about 74 percent of the anions (as milliequivalents per liter). The Colorado Department of Health has not established water-quality standards for the major chemical constituents for water use from Lake Henry. However, dissolved-solids concentration and the per- centage of individual cations and anions are important factors in water used for agricultural purposes. Some investigators have recommended an upper limil of dis- solved solids of about 5,000 mglL for water to be used by livestock (Hem, 1985). Many crops are sensitive to large concentrations of dissolved solids. Dissolved- solids concentrations from 500 to 1.000 mglL can have detrimental effects on sensitive crops. Dissolved- sol ids concentrations from 1,000 to 2,000 mglL may have adverse effects on many crops (National Acad- emy of Sciences, National Academy of Engineering, 1973). Water Ihat has a large proportion of sodium to total cations is not suitable as irrigation water. A large sodium content is toxic to some plants and causes the soil to become impermeable to water. The sodium- adsorption ratio of water is used to predict the degree to which sodium may replace adsorbed calcium and mag- nesium. The sodium-adsorption ratio is calculated using the following equation: Sodium-adsorption ratio = Sodium JI/2[calcium+magnesium] where sodium, calcium, and magnesium are expressed in milliequivalents per liter. Large sodium-adsorption- ratio values imply a hazard of sodium replacing adsorbed calcium and magnesium, a situation that may damage the soil structure (Hem, 1985). The sodium- adsorption ratio for water samples analyzed in Lake Henry averaged about 1.4. Water that has a sodium- adsorption-ratio value less than 4 is safe for irrigating fruit; whereas, for most other crops, sodium- adsorption-ratio values that range from 8 to 18 are considered safe (National Academy of Sciences. National Academy of Engineering. 1973). The Colorado Department of Health (I 987) has established water-quality standards for selected trace elements for Lake Henry. On August 19 and October 6, concentrations of total-recoverable iron at site HEW2 exceeded the water-quality standard for aquatic life of 1.000 IlgfL. On August 19, total- recoverable iron concentrations were 1,000 IlgfL near the lake surface and 6.300 IlgfL near the lake bOllom; on October 6, concentrations of total-recoverable iron were 1,900 IlglL near the lake surface and 2,500 IlgfL near the lake bOllom. Concentrations of other trace ele- ments were less than the water-quality standards or less than the detection limits for the analysis (table 16). 16 Reconnaissance of Water Quality of Lake Henry and lake Meredith Reservoir, Crowley County, Southeastern Colorado, April-October 1987